WASHINGTON — If President Trump is worried that he could be impeached should Democrats take control of the House in the midterm elections, he is not acting like it. If anything lately, he seems to be offering more examples for his opponents to use against him.

His tweet over the holiday weekend chastising Jeff Sessions, the attorney general, for the Justice Department’s recent indictments of two Republican congressmen because it could cost the party seats in November crossed lines that even he had not yet breached, asserting that specific continuing criminal prosecutions should be decided on the basis of partisan advantage.

Shocking as many legal and political figures found it — one Republican senator compared it to “banana republic” thinking — the message by itself might not rise to the level of high crimes and misdemeanors required for impeachment because it could be construed as commentary rather than an order. But legal scholars and some lawmakers said it could be one more exhibit in trying to prove a pattern of obstruction or reckless disregard for the rule of law in a future impeachment proceeding.

Senator Sheldon Whitehouse, Democrat of Rhode Island and a former United States attorney, said “rare is the case that you make with one item of evidence,” but the president’s comment could be “potentially additional evidence of corrupt intent in an obstruction of justice prosecution.”

The tweet got lost to some extent amid the Supreme Court confirmation hearings of Brett M. Kavanaugh and the new Bob Woodward book filled with titillating anecdotes from inside Mr. Trump’s White House. But it further cemented the impression of a president who sees the law enforcement agencies that report to him as political instruments rather than semiautonomous organs that should remain free of partisan influence.

Over nearly 20 months in office, Mr. Trump has repeatedly castigated the Justice Department and the F.B.I. for investigating his associates and not investigating his enemies. He has threatened time and again to fire Mr. Sessions because his recusal from the Russia investigation meant that he could not protect the president from the inquiry.

Mr. Woodward’s book, “Fear,” quotes Mr. Trump excoriating Mr. Sessions in particularly personal terms. “This guy is mentally retarded. He’s this dumb Southerner,” the president was said to tell advisers. “How in the world was I ever persuaded to pick him for my attorney general? He couldn’t even be a one-person country lawyer down in Alabama.”

In a late-night tweet, Mr. Trump denied calling Mr. Sessions either “mentally retarded” or “a dumb southerner,” accusing Mr. Woodward of fabricating the quotes. “I said NEITHER, never used those terms on anyone, including Jeff, and being a southerner is a GREAT thing,” Mr. Trump wrote. “He made this up to divide!”

The post Mr. Trump wrote on Monday took his criticism of the Justice Department to the next step, suggesting that defending the Republican majority in the House should determine whether two members are prosecuted.

“Two long running, Obama era, investigations of two very popular Republican Congressmen were brought to a well publicized charge, just ahead of the Mid-Terms, by the Jeff Sessions Justice Department,” he wrote. “Two easy wins now in doubt because there is not enough time. Good job Jeff.”

He was presumably referring to indictments of Representatives Duncan Hunter of California, who was charged with using more than $250,000 in campaign funds for personal expenses, and Chris Collins of New York, who was charged with insider trading. Both have pleaded not guilty.

Mr. Trump’s suggestion would have been a major scandal under any other president, veterans of past administrations said. “His interference in an ongoing criminal investigation may be the single most shocking thing he’s done as president,” said Walter E. Dellinger, a former acting solicitor general under President Bill Clinton.

Senator Jeff Flake of Arizona, a Republican who has been among the president’s most outspoken critics in his own party, had the same reaction. “Those who study this kind of thing say it’s a lot more evidence for abuse of power or obstruction,” he said. “I just know it’s not healthy for the institutions of government to have the president want to use the Department of Justice that way.”

Senator Lisa Murkowski, Republican of Alaska, likewise criticized the president’s comments. “I’m looking at them just as you are looking at them,” she told reporters. “I thought that yesterday’s comments were not appropriate and they upset me.”

Asked if Mr. Trump was only feeding Democrats’ interest in impeachment, she said: “I have no idea what he is doing. I have no idea what he is thinking.”

Other Republicans avoided commenting or seemed less concerned. “I have no reason to believe these prosecutions are politically motivated,” said Senator Lindsey Graham of South Carolina. “I think it was a comment not designed to obstruct justice but in my view, not appropriate.”

As has increasingly become the pattern, the White House held no briefing for reporters on Tuesday and a spokesman for the president did not respond to a request for comment.

But David B. Rivkin, a Washington lawyer and veteran of past Republican administrations, said it was fair to question why the Justice Department charged the two congressmen so close to an election when the past three attorneys general signed memos discouraging such prosecutions during a campaign season.

“What is missing in this whole controversy is D.O.J. derogation from decades-long policy from not pursuing any overt investigative actions against politicians close to the elections,” Mr. Rivkin said. “Trump has a salient point even if he didn’t phrase it well, making it seem like he is objecting to the indictments of those individuals, period.”

Mr. Trump seems so intent on pushing the boundaries when it comes to the nation’s law enforcement agencies that some wonder whether he is almost baiting Democrats to pursue impeachment against him — either because he wants it to be an issue in the midterm elections to motivate his conservative base to turn out to defend him or to delegitimize any postelection impeachment effort as a strictly partisan affair.

The president has been told many times that tweets like the one over the weekend could expose him to trouble, if not from Robert S. Mueller III, the special counsel investigating Russian interference in the 2016 election, then from Democrats, who can interpret the constitutional phrase “high crimes and misdemeanors” as they choose if they take power.

Gwenda Blair, a Trump biographer, said his persistence in weighing in on law enforcement actions represented a defiance of the establishment that keeps telling him he should not do so. “He’s not going to kowtow to anybody, there are no sacred cows that he won’t move out of the way, and that still apparently resonates,” she said.

If Mr. Sessions had actually done what Mr. Trump suggested — refused to indict to protect Republican seats in the election — it could be considered a crime or an impeachable offense for the attorney general. But because Mr. Trump merely offered his opinion, as opposed to issuing an order, some legal scholars said it might be hard to define that as an impeachable offense by the president.

“Impeaching on this basis would be extraordinarily difficult because he would have available to him the defense that it was public commentary and not a directive to the attorney general,” said Joshua Matz, a co-author of “To End a Presidency: The Power of Impeachment” and publisher of Take Care, a blog that follows legal issues raised by Mr. Trump’s presidency.

But impeachment advocates could include it as one more point among the president’s many tweets and comments on law enforcement to demonstrate an intent to abuse his authority.

“To the extent that they can all be strung together convincingly as a pattern of using the power of government to destroy political opponents,” Mr. Matz said, “that might well justify serious consideration of articles of impeachment.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: With a Barb, Trump Blurs A Legal Line. Order Reprints | Today’s Paper | Subscribe